<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>JS的变量</title>

    <script>
    // 定义变量
    /* var a = 3;
    alert(a);

    a = "abc";
    alert(a); */

    // 定义 number 类型
    var num = 1;
    var num2 = 1.1;
    var num3 = NaN;

    // 输出到页面上
    document.write(num+"<br>");
    document.write(num2+"<br>");
    document.write(num3+"<br>");

    // 定义 string 类型
    var str = "abc";
    var str2 = 'a';
    document.write(str + "<br>");
    document.write(str2 + "<br>");

    // 定义 boolean 类型
    var flag = true;
    var flag2 = false;
    document.write(flag + "<br>");
    document.write(flag2 + "<br>");

    // 定义 null 类型
    var obj = null;
    var obj2 = undefined;
    var obj3;
    document.write(obj + "<br>");
    document.write(obj2 + "<br>");
    document.write(obj3 + "<br>");

    </script>

</head>
<body>

</body>
</html>